Is interchangeability a word? I guess so since there's no squiggly red line under it. Anyhoo, while I touched on this stuff in the compiled brake information thread, I thought maybe it couldn't hurt to have it all laid out a little more clearly.
Basically, most Subaru brakes will interchange onto other Subarus. That's very convenient when you want to put bigger brakes on your non-STi Subaru without spending a lot of money. But, when talking about physically bolting the brakes up to another Subaru, there are a few things to be concerned about:
1. Wheel Clearance
2. Rotor bolt pattern
3. Caliper mounting
4. Rear parking brake diameter
#1 is pretty simple. If you're putting brembos on your 93 Impreza L, obviously the the tiny stock steelies aren't going to fit.
#2 is something you have to be concerned with when installing brembos. The 05+ STi has a 5x114.3 bolt pattern. So while you can bolt the calipers right up other cars, you'll need to use rotors drilled in 5x100, such as those on the 04 sti. The Tribeca and SVX also have a 5x114.3 bolt pattern, while all the other Subarus since 1990 or so are 5x100 (well, except the Justy and Loyale).
#3 comes into play when installing STi brembos or the Subaru 2-pots in the REAR. Any FRONT caliper/bracket will bolt up to the front of any other modern Subaru (except the SVX), but in the rear there are different backing plates. The backing plate is attached to the rear spindles, and is what the calipers bolt to. There are a few different backing plates, and brakes will not interchange between them. The solution is to either switch backing plates or use an adapter bracket.
The way to divide up the backing plates is as follows:
-Drums
-Sliding 1-pot calipers
-Fixed 2-pot calipers
-08+ Imprezas
Obviously you can't bolt discs up to a drum backing plate since there's nowhere for the caliper to attach. To swap from drums you'll need to remove the brakes, remove the backing plates, and install disc backing plates , parking brakes (also the cables) and then rotors and calipers. Generally it's easier to find some disc spindles and swap the whole things over.
For the sliding calipers, almost everything is interchangeable. That includes everything from a 1990 Legacy to Imprezas and Tribecas and even the SVX (although bolt patterns may vary).
To bolt a 2-pot rear caliper to a 1-pot car, you'll need to install either the 2-pot backing plates, which involves removing the parking brake, entire spindle, and using a press, or you can purchase some adapter brackets. Tom@Kartboy manufactures brackets to attach either the Subaru 2-pots or 04-07 Brembos to the rear of any other Subaru.
If you want to swap rear brembos for the 2-pots, or vice-versa, no bracket is required. Sti brembos will bolt right up to an 06-07 WRX, and the rear 2-pots will bolt right up to an 04-07 STi.
If, for some reason, you wanted some sliding calipers on your 2-pot car, you're out of luck. No bracket exists so you'll have to pull off the spindles and swap backing plates.
For 2008, the Impreza has a whole different thing going on in the rear. While nothing changed with the front brakes, the rear suspension, spindles, and caliper mounting changed on regular Imprezas, the WRX, and the STi. So to the best of my knowledge, the 08 rear brembos won't swap to anything else (although there's a slim chance it's possible to put them on a 08 WRX).
When it comes to STi Brembos, there's another little issue that arises when switching brakes and that is the rear parking brake. The STi, along with a few non-US models, has a larger 190mm parking brake drum (compared to 170mm on everything else). That means if you were to pick up some rear STi brembo calipers and rotors and put them on your non-sti, the parking brake wouldn't work because there's a 10mm gap between the drum and the shoes. It also means if, for rally purposes perhaps, you wanted some Subaru 2-pots on your STi, the rotors wouldn't fit over the parking brake.
Thankfully there's a solution for each situation. Subaru took care of the Brembo -> 2-pot swap because the 2-pots are the group N rally-spec brakes. So they make rotors that work with the 2-pots but fit over STi parking brakes, in both 5x100 and 5x114.3. You can get them from rally shops such as Rallispec or Rocket Rally.
To make rear brembos work on a 170mm parking brake car, there are a couple of solutions. The best one is the DBA 2657 rotor. It's a brembo-sized rotor, drilled for 5x100, with a 170mm parking brake hat. This is a new thing that was produced specifically to go with the Kartboy Brembo adapter brackets, so be sure to thank Tom and DBASteve for getting together on that.
Before then, the solution was to either swap to a bigger parking brake, get some spacers made to fit in the rotor hat, or buy some taller parking brake shoes. Godspeed in the UK makes taller shoes, but it will cost you more than the DBA rotors.
Another thing to note is that the 08 WRX has the same larger 190mm parking brake hat as the STi.
Okay, well, I think that covers it. In summary, pretty much everything interchanges when it comes to front brakes, make sure rotors have the right bolt pattern, and things get a bit complicated when it comes to the rear.
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Pretty sure STI rotors won't work with WRX brake caliper or brackets. As you point out there's an 8mm difference in width of the rotor and it's taller than the WRX rotor by a fairish amount.
STI rotors are closer to Legacy GT rotors at 30mm thick. I believe the STI rotors are about 1/2" taller than the LGT rotors even.
LGT rotors with LGT/Tribeca calipers and brackets are the hot ticket for front brakes. Just a little less capable than brembo's. It's a straight bolt on too. LGT brakes will kill the subaru 4 pots or standard 2 pot brakes. That's what I had on my car till...yesterday when I installed Brembo's.
